L. R. Picard, M. J. Mark, F. Ferlaino, R. van Bijnen Deep Learning-Assisted Classification of Site-Resolved Quantum Gas Microscope Images,
Measurement Science and Technology 31 25201 (2019-11-05),
http://dx.doi.org/10.1088/1361-6501/ab44d8 doi:10.1088/1361-6501/ab44d8 (ID: 720262)
C. R. Kaubrügger, P. Silvi, C. Kokail, R. van Bijnen, A. M. Rey, J. Ye, A. Kaufman, P. Zoller Variational spin-squeezing algorithms on programmable quantum sensors,
Phys. Rev. Lett. 123 260505 (2019-08-22),
http://dx.doi.org/10.1103/PhysRevLett.123.260505 doi:10.1103/PhysRevLett.123.260505 (ID: 720356)
Arrays of atoms trapped in optical tweezers combine features of programmable analog quantum simulators with atomic quantum sensors. Here we propose variational quantum algorithms, tailored for tweezer arrays as programmable quantum sensors, capable of generating entangled states on-demand for precision metrology. The scheme is designed to generate metrological enhancement by optimizing it in a feedback loop on the quantum device itself, thus preparing the best entangled states given the available quantum resources. We apply our ideas to generate spin-squeezed states on Sr atom tweezer arrays, where finite-range interactions are generated through Rydberg dressing. The complexity of experimental variational optimization of our quantum circuits is expected to scale favorably with system size. We numerically show our approach to be robust to noise, and surpassing known protocols.
G. Natale, R. van Bijnen, A. Patscheider, D. Petter, M. J. Mark, L. Chomaz, F. Ferlaino Excitation spectrum of a trapped dipolar supersolid and its experimental evidence,
Phys. Rev. Lett. 123 50402 (2019-08-01),
http://dx.doi.org/10.1103/PhysRevLett.123.050402 doi:10.1103/PhysRevLett.123.050402 (ID: 720313)
We study the spectrum of elementary excitations of a trapped dipolar Bose gas across the superfluid-supersolid phase transition. Our calculations, accounting for the experimentally relevant case of confined systems, show that, when entering the supersolid phase, two distinct excitation branches appear, respectively connected to crystal or superfluid orders. These results confirm infinite-system predictions, showing that finite-size effects play only a small qualitative role. Experimentally, we probe compressional excitations in an Er quantum gas across the phase diagram. While in the BEC regime the system exhibits an ordinary quadrupole oscillation, in the supersolid regime, we observe a striking two-frequency response of the system, related to the two spontaneously broken symmetries.
C. Kokail, C. Maier, R. van Bijnen, T. Brydges, M. K. Joshi, P. Jurcevic, C. A. Muschik, P. Silvi, R. Blatt, C. F. Roos, P. Zoller Self-verifying variational quantum simulation of lattice models,
Nature 569 360 (2019-05-15),
http://dx.doi.org/10.1038/s41586-019-1177-4 doi:10.1038/s41586-019-1177-4 (ID: 720076)
Hybrid classical-quantum algorithms aim at variationally solving optimisation problems, using a feedback loop between a classical computer and a quantum co-processor, while benefitting from quantum resources. Here we present experiments demonstrating self-verifying, hybrid, variational quantum simulation of lattice models in condensed matter and high-energy physics. Contrary to analog quantum simulation, this approach forgoes the requirement of realising the targeted Hamiltonian directly in the laboratory, thus allowing the study of a wide variety of previously intractable target models. Here, we focus on the Lattice Schwinger model, a gauge theory of 1D quantum electrodynamics. Our quantum co-processor is a programmable, trapped-ion analog quantum simulator with up to 20 qubits, capable of generating families of entangled trial states respecting symmetries of the target Hamiltonian. We determine ground states, energy gaps and, by measuring variances of the Schwinger Hamiltonian, we provide algorithmic error bars for energies, thus addressing the long-standing challenge of verifying quantum simulation.
D. Petter, G. Natale, R. van Bijnen, A. Patscheider, M. J. Mark, L. Chomaz, F. Ferlaino Probing the roton excitation spectrum of a stable dipolar Bose gas,
Phys. Rev. Lett. 122 183401 (2019-05-08),
http://dx.doi.org/10.1103/PhysRevLett.122.183401 doi:10.1103/PhysRevLett.122.183401 (ID: 720098)
We measure the excitation spectrum of a stable dipolar Bose--Einstein condensate over a wide momentum-range via Bragg spectroscopy. We precisely control the relative strength, εdd, of the dipolar to the contact interactions and observe that the spectrum increasingly deviates from the linear phononic behavior for increasing εdd. Reaching the dipolar dominated regime εdd>1, we observe the emergence of a roton minimum in the spectrum and its softening towards instability. We characterize how the excitation energy and the strength of the density-density correlations at the roton momentum vary with εdd. Our findings are in excellent agreement with numerical calculations based on mean-field Bogoliubov theory. When including beyond-mean-field corrections, in the form of a Lee-Huang-Yang potential, we observe a quantitative deviation from the experiment, questioning the validity of such a description in the roton regime.
L. Chomaz, D. Petter, P. Ilzhöfer, G. Natale, A. Trautmann, C. Politi, G. Durastante, R. van Bijnen, A. Patscheider, M. Sohmen, M. J. Mark, F. Ferlaino Long-Lived and Transient Supersolid Behaviors in Dipolar Quantum Gases,
Phys. Rev. X 9 21012 (2019-04-19),
http://dx.doi.org/10.1103/PhysRevX.9.021012 doi:10.1103/PhysRevX.9.021012 (ID: 720203)
By combining theory and experiments, we demonstrate that dipolar quantum gases of both 166Er and 164Dy support a state with supersolid properties, where a spontaneous density modulation and a global phase coherence coexist. This paradoxical state occurs in a well defined parameter range, separating the phases of a regular Bose-Einstein condensate and of an insulating droplet array, and is rooted in the roton mode softening, on the one side, and in the stabilization driven by quantum fluctuations, on the other side. Here, we identify the parameter regime for each of the three phases. In the experiment, we rely on a detailed analysis of the interference patterns resulting from the free expansion of the gas, quantifying both its density modulation and its global phase coherence. Reaching the phases via a slow interaction tuning, starting from a stable condensate, we observe that 166Er and 164Dy exhibit a striking difference in the lifetime of the supersolid properties, due to the different atom loss rates in the two systems. Indeed, while in 166Er the supersolid behavior only survives a few tens of milliseconds, we observe coherent density modulations for more than 150ms in 164Dy. Building on this long lifetime, we demonstrate an alternative path to reach the supersolid regime, relying solely on evaporative cooling starting from a thermal gas.
J. Zeiher, J. Choi, A. Rubio Abadal, T. Pohl, R. van Bijnen, I. Bloch, C. Gross Coherent many-body spin dynamics in a long-range interacting Ising chain,
Phys. Rev. X 7 41063 (2017-12-14),
http://dx.doi.org/10.1103/PhysRevX.7.041063 doi:10.1103/PhysRevX.7.041063 (ID: 719831)
Coherent many-body quantum dynamics lies at the heart of quantum simulation and quantum computation. Both require coherent evolution in the exponentially large Hilbert space of an interacting many-body system. To date, trapped ions have defined the state of the art in terms of achievable coherence times in interacting spin chains. Here, we establish an alternative platform by reporting on the observation of coherent, fully interaction-driven quantum revivals of the magnetization in Rydberg-dressed Ising spin chains of atoms trapped in an optical lattice. We identify partial many-body revivals at up to about ten times the characteristic time scale set by the interactions. At the same time, single-site-resolved correlation measurements link the magnetization dynamics with inter-spin correlations appearing at different distances during the evolution. These results mark an enabling step towards the implementation of Rydberg atom based quantum annealers, quantum simulations of higher dimensional complex magnetic Hamiltonians, and itinerant long-range interacting quantum matter. (local copy)
J. Cui, R. van Bijnen, T. Pohl, S. Montangero, T. Calarco Optimal control of Rydberg lattice gases,
Quantum Sci. Technol. 2 35006 (2017-08-02),
http://dx.doi.org/10.1088/2058-9565/aa7daf doi:10.1088/2058-9565/aa7daf (ID: 719830)
We present optimal control protocols to prepare different many-body quantum states of Rydberg atoms in optical lattices. Specifically, we show how to prepare highly ordered many-body ground states, GHZ states as well as some superposition of symmetric excitation number Fock states, that inherit the translational symmetry from the Hamiltonian, within sufficiently short excitation times minimizing detrimental decoherence effects. For the GHZ states, we propose a two-step detection protocol to experimentally verify the optimal preparation of the target state based only on standard measurement techniques. Realistic experimental constraints and imperfections are taken into account by our optimization procedure making it applicable to ongoing experiments. (local copy)
A. Glätzle, R. van Bijnen, P. Zoller, W. Lechner A Coherent Quantum Annealer with Rydberg Atoms,
Nat. Commun. 8 15813 (2017-06-22),
http://dx.doi.org/10.1038/ncomms15813 doi:10.1038/ncomms15813 (ID: 719686)
There is a significant ongoing effort in realizing quantum annealing with different physical platforms. The challenge is to achieve a fully programmable quantum device featuring coherent adiabatic quantum dynamics. Here we show that combining the well-developed quantum simulation toolbox for Rydberg atoms with the recently proposed Lechner-Hauke-Zoller~(LHZ) architecture allows one to build a prototype for a coherent adiabatic quantum computer with all-to-all Ising interactions and, therefore, a novel platform for quantum annealing. In LHZ a infinite-range spin-glass is mapped onto the low energy subspace of a spin-1/2 lattice gauge model with quasi-local 4-body parity constraints. This spin model can be emulated in a natural way with Rubidium and Cesium atoms in a bipartite optical lattice involving laser-dressed Rydberg-Rydberg interactions, which are several orders of magnitude larger than the relevant decoherence rates. This makes the exploration of coherent quantum enhanced optimization protocols accessible with state-of-the-art atomic physics experiments. (local copy)